[Eeglablist] ERP analyses and average referencing

Thu Oct 23 15:16:10 PDT 2008

I'm surprised not to see a free-reference option like the laplassian
transform here.

On Thu, Oct 23, 2008 at 7:29 PM, Steve Luck <sjluck at ucdavis.edu> wrote:

> I'd like to make one last set of comments about the choice of the reference
> electrode:
>
> 1) Alex is right that there is no truly correct reference.  It is always a
> matter of balancing the advantages and disadvantages of different reference
> montages.
>
> 2) He is also correct that there may be statistical power advantages to
> using the average reference.
>
> 3) The major advantage of mastoids (and earlobes, etc.) is that they are
> widely used in ERP research (although not in all subareas).
>
> 4) The most important thing is to recognize that your data are always
> influenced by your choice of reference.  As long as you don't forget this
> (and make sure that others don't forget it as well), you will be fine.
>
> 5) For experiments with broad, dense, and uniform distributions of
> electrodes, the wisest choice is usually to look at the data _both_ with an
> average reference and with a mastoid (or earlobe, etc.) reference.  That
> gives you the best of both worlds.
>
> 6) For experiments with a limited or nonuniform distribution of electrodes,
> avoid using the average reference (unless you are being extremely careful).
>
> Steve Luck
>
> *From: *"Alexander J. Shackman" <shackman at wisc.edu>
> *Date: *October 21, 2008 12:22:47 PM PDT
> *To: *eeglablist at sccn.ucsd.edu
> *Subject: **Re: [Eeglablist] ERP analyses and average referencing*
> *Reply-To: *ajshackman at gmail.com
>
>
> Arno prefaced his comments by noting that "average referencing is always
> incorrect." But as Joe Dien notes in his excellent '98 paper, it would be
> equally appropriate to say that "mastoids montages are always incorrect" or
> "ALL referencing schemes are always incorrect."
> Steve and Arno are correct in noting, as Dien did, that the topography and
> waveforms will differ across montages, making it difficult to compare
> average to the more commonly used (for ERPs, at least) mastoids montage.
>
> But there are at least two other reasons, at least with high-density
> recordings, to consider using the average reference. 1) Dien suggests that
> the average reference, which is employed by both dipolar and distributed
> source modeling algorithms, potentially provides more insight into the
> underlying cerebral generators. 2) The average reference is likely to more
> psychometrically reliable (cf. S. Gudmundsson et al., Clinical Neurophys,
> 2007).
>
> Alex Shackman
>
>
>
>
> On Sun, Oct 19, 2008 at 2:29 PM, Steve Luck <sjluck at ucdavis.edu> wrote:
>
>> I would like to echo and expand upon Arno's comments about average
>> referencing.  Under the most optimal conditions this can be perhaps a decent
>> approximation (see Dien, 1998).  However, under most conditions it is a poor
>> and misleading approximation (and, as Arno pointed out, is is never
>> completely correct).  Your waveforms will look completely different
>> depending on what electrodes you happen to be using (see Figure 2 and the
>> related text in chapter 3 of An Introduction the Event-Related Potential
>> Technique).  As a result, your data may look quite different from the data
>> of other researchers, even if they are also using the average of all sites
>> as the reference (because they probably don't have exactly the same set of
>> sites that you have).
>>
>> So, what to do?  Lately, my lab has been seeing the same sort of problem,
>> with lots of muscle activity being picked up by mastoid reference
>> electrodes.  The best thing to do is to try to get subjects to sit in a more
>> neutral position so that they do not need to contract the neck muscles to
>> keep the head upright. However, if you already have this noise in your
>> mastoid data, you can try referencing to scalp sites that are close to the
>> mastoids (e.g., P9 and P10), which may have less muscle noise.  Or, if you
>> have a sufficiently dense array of electrodes, you could use the average of
>> a small cluster around the mastoids on each side as the reference.
>>
>> The most important thing is to realize that you are _always_ looking at
>> the potential between two electrode sites (or groups of sites).  There is no
>> such thing as potential at a single site.
>>
>> Steve Luck
>>
>> *From: *arno delorme <arno at ucsd.edu>
>> *Date: *October 18, 2008 4:15:53 AM PDT
>> *To: *Yvonne Tran <Yvonne.Tran at uts.edu.au>
>> *Cc: *eeglablist at sccn.ucsd.edu
>> *Subject: **Re: [Eeglablist] ERP analyses and average referencing*
>>
>>
>> Dear Yvonne,
>>
>> average referencing is always incorrect. The amount of current going in
>> and out of the head is assumed to be 0. Using that properties, average
>> referencing means that the average potential across all electrode is 0 at
>> all times. However, you cannot expect that the electrode spatial
>> distribution will be homogenous over the head (because first you cannot put
>> any within the neck, and there is usually no electrode on the face etc...).
>> It is generally assumed that the current flowing within the neck is
>> negligible (because of high conductances).
>>
>> As an answer to your question, if your electrode repartition is relatively
>> homogenous on the scalp, then you may use average reference. Nevertheless,
>> average reference will not make it easy to compare between montages.
>>
>> Best regards,
>>
>> Arno
>>
>> On 17 sept. 08, at 04:20, Yvonne Tran wrote:
>>
>> Dear All
>>
>>
>> We are currently working with spinal cord injured participants and have
>> recorded some oddball data. We have been using A1 and A2 mastoid for
>> reference channels, however, with this particular group we are experiencing
>> increased muscle tension in this region (which cannot be prevented, as some
>> participants are unaware that they are tensing up), and therefore when the
>> data are re-referenced the other EEG channels become flooded with muscle
>> tension noise. This can be overcome when we re-reference using average
>> referencing. My question is how many electrodes (evenly distributed around
>> the scalp) will be ok for average referencing for ERP analyses? We have 26
>> EEG channels.
>>
>>
>> Any suggestions/opinions appreciated!
>>
>>
>> Thank you
>>
>> regards
>>
>> Yvonne
>>
>>
>> --------------------------------------------------------------------
> Steven J. Luck, Ph.D.
> Professor
> Center for Mind & Brain and Department of Psychology
> University of California, Davis
> 267 Cousteau Place
> Davis, CA 95618
> (530) 297-4424
> sjluck at ucdavis.edu
> --------------------------------------------------------------------
>
>
>
> _______________________________________________
> Eeglablist page: http://sccn.ucsd.edu/eeglab/eeglabmail.html
> To unsubscribe, send an empty email to
> eeglablist-unsubscribe at sccn.ucsd.edu
> For digest mode, send an email with the subject "set digest mime" to
> eeglablist-request at sccn.ucsd.edu
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20081024/c54e3278/attachment.html>